This invention relates generally to signal processing systems and more particularly to beamforming controls for phased array antennas.
Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antennas (or subarrays of antennas), can be accomplished by electronically shifting the phases of the actuating signals or by introducing a time delay in the different actuating signals to sequentially excite the antenna elements to generate the desired direction of beam transmission from the antenna.
Electronically shifting the phases of the actuating signals requires extensive equipment, including switching devices to route the electrical signals through appropriate hardwired circuits to achieve the desired phase changes. Electronic phase shifters are designed for use at a specific frequency and thus have significant drawbacks when employed in phased array antenna systems using broad band radiation. For example, most hardwired phase shifters are limited to frequency changes of 1% or less of the design frequency of the shifter in order to avoid beam squint, or the variation from the beam direction that would result with the same phase delay at the design frequency.
Optical control systems can be advantageously used to create selected time delays in actuating signals for phased array systems. Such optically generated time delays are not frequency dependent and thus can be readily applied to broadband phased array antenna systems. For example, optical signals can be processed to establish the selected time delays between individual signals to cause the desired sequential actuation of the transmitting antenna elements, and the optical signals can then be converted to electrical signals, such as by a photodiode array. Different types of optical architectures have been proposed to process optical signals to generate selected delays, such as routing the optical signals through optical fiber segments of different lengths; using deformable mirrors to physically change the distance light travels along a reflected path before being converted to an electrical signal; and utilizing free space propagation based delay lines, which architecture typically incorporates polarizing beam splitters and prisms.
The use of optical fiber segments to introduce delays requires the use of many optical switches and the splicing of numerous segments of fiber together. The costs of construction of such a device are substantial given the significant amount of design work and precision assembly work necessary to produce a device having the range and incremental steps of phase changes that are required in a typical system, such as for a phased array radar. The numerous switching and coupling elements also introduce very high optical losses in the beamforming circuitry, requiring significant optical power input. The structure of the circuitry makes it less compact and less rugged than other types of systems discussed below.
The deformable mirror system relies on the physical displacement of a mirror to effect the necessary time delay; an array of moveable mirrors allows the generation of a range of delayed optical signals. This type of system is less rugged and potentially prone to calibration errors given the requirement displacement of the mirror to achieve the small time delays required for the optical signals.
An optical architecture for a transmit-only control circuit utilizing coherent light in conjunction with free space delay units was proposed by D. Dolfi, F. Michel-Gabriel, S. Bann, and J. Huignard in the paper entitled "Two-dimensional optical architecture for time-delay beam forming in a phased-array antenna", Vol. 16, Optics Letters, pp. 255-57, Feb. 15, 1991. The system proposed by Dolfi utilizes a coherent beam of light from a laser which is directed through a cascade of free space delay devices comprising spatial light modulators, polarizing beam splitters and prisms. By selectively polarizing various light beams from the laser, the beams can be individually directed through one or more of the free space delay devices to introduce a time delay to the beam. The delayed beams are ultimately directed through an array of microlenses to photodiodes which convert the optical signals into electrical signals to actuate the transmission antenna. Dolfi does not suggest the use of his device for processing signals from returned beams detected by the antenna. Additionally, the use of coherent light necessitates the use of high quality optical components in the system to maintain the coherence of the light from the laser source in order to modulate the laser beam by interference between two coherent beams. Given the sensitivity of such components to motion, this type of a system is less rugged than systems relying on incoherent light, which do not use the interference phenomenon.
It is accordingly a primary object of this invention to provide an optical beamforming architecture that can both process signals to control beams transmitted from a phased array antenna and process signals from return beams detected by a phased array antenna.
It is another object of the present invention to provide an optical beamforming architecture that has low optical losses, and that is compact and rugged.
It is a further object of this invention to provide an optical architecture that can be operated with either incoherent or coherent optical signals.